A. Field of the Invention
This invention relates to digital waveform processing and, more particularly, to the deskewing and delaying of a waveform under digital control.
B. Description of the Prior Art
Digitally-controlled capacitive devices may serve a number of uses which include being used as digitally-controlled delays in clock generator circuits. The digitally-controlled delays are necessary in such circuits for precision deskewing of output waveforms. Circuit implementations of the digitally controlled delays have generally involved attaching a capacitive load to a signal line with a transmission gate coupled between the capacitive load and the signal line. The signal line carries the waveform to be delayed and the transmission gate acts as a digital switch to selectively connect or disconnect the capacitive load to the signal line. When the capacitive load is connected to the signal line, the waveform carried by the signal line is correspondingly delayed. Conversely, when the capacitive load is disconnected from the signal line, the waveform carried by the signal line is not delayed.
Prior art implementations of digitally-controlled capacitive loads suffer from the problems associated with the transmission gate used as a switch to connect and disconnect the capacitive load to the signal line. In general, the prior art transmission gate itself adds a significant amount of capacitance to the signal line, even when the gate is switched off. Moreover, the prior art transmission gate is known to cause glitches on the signal line each time the gate is switched. Considering objectionable characteristics in the prior art digitally-controlled capacitive loads, an improved digitally-controlled capacitive load is desired.